A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical-pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 40 percent.
The use of flashlamps, light-emitting diodes (as used herein, this term includes superluminescent diodes and superluminescent diode arrays) and laser diodes (as used herein, this term includes laser diode arrays) to optically pump or excite a solid lasant material is wellknown. Lasant materials commonly used in such solid state lasers include crystalline or glassy host materials into which an active material, such as trivalent neodymium ions, is incorporated. Highly suitable solid lasant materials include substances wherein the active material is a stoichiometric component of the lasant material. Such stoichiometric materials include, for example, neodymium pentaphosphate and lithium neodymium tetraphosphate. Detailed summaries of conventional solid lasant materials are set forth in the CRC Handbook of Laser Science and Technology, Vol. I, M. J. Weber, Ed., CRC Press, Inc., Boca Raton, Florida, 1982, pp. 72-135 and by A. A. Kaminskii in Laser Crystals, Vol. 14 of the Springer Series in Optical Sciences, D. L. MacAdam, Ed., Springer-Verlag, New York, N.Y., 1981. Conventional host materials for neodymium ions include glass, yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12, referred to as YAG), YAlO.sub.3 (referred to as YALO), LiYF.sub.4 (referred to as YLF), and gadolinium scandium gallium garnet (Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12, referred to as GSGG). By way of example, when neodymium-doped YAG is employed as the lasant material in an optically-pumped solid state laser, it can be pumped by absorption of light having a wavelength of about 808 nm and can emit light having a wavelength of 1064 nm.
U.S. Pat. No. 3,624,545 issued to Ross on Nov. 30, 1971, describes an optically-pumped solid state laser composed of a YAG rod which is side-pumped by at least one semiconductor laser diode. Similarly, U.S. Pat. No. 3,753,145 issued to Chesler on Aug. 14, 1973, discloses the use of one or more light-emitting semiconductor diodes to end-pump a neodymium-doped YAG rod. The use of an array of pulsed laser diodes to end-pump a solid lasant material such as neodymium-doped YAG is described in U.S. Pat. No. 3,982,201 issued to Rosenkrantz et al. on Sept. 21, 1976. Finally, D. L. Sipes, Appl. Phys. Lett., Vol. 47, No. 2, 1985, pp. 74-75, has reported that the use of a tightly focused semiconductor laser diode array to end pump a neodymium-doped YAG results in a high efficiency conversion of pumping radiation having a wavelength of 810 nm to output radiation having a wavelength of 1064 nm.
Solid-state lasers which exhibit single longitudinal mode operation to yield a single-frequency output can be obtained by eliminating spatial hole burning in the lasant material. Spatial hole burning is a consequence of the electric field nodes that are associated with a linearly polarized standing wave. The population inversion in the lasant material at these nodes does not contribute to the standing wave and will preferentially contribute to other longitudinal modes. In gas lasers, spatial hole burning is substantially prevented by the thermal motion of the atoms and/or molecules of the gas. However, in solids the motion of atoms and/or molecules is small in comparison with the wavelength of the light produced by the laser, and spatial averaging cannot take place.
Spatial hole burning in a solid-state laser can be prevented by eliminating the accumulation of an unutilized population inversion in the lasant material at the nodes of a standing wave. As a consequence, alternative longitudinal modes are prevented from reaching threshold, and the laser can produce single-frequency output through single longitudinal mode operation. For example, spatial hole burning can be eliminated through the use of a unidirectional ring cavity, by generating circularly polarized light in the lasant material, with mechanical motion, or with electro-optic phase modulations.
The use of a unidirectional ring cavity to prevent spatial hole burning is summarized in W. Koechner, Solid-State Laser Engineering (Springer-Verlag, New York, Second Ed., 1988) at pp., 126-128 and 223, and in Siegman, Lasers (University Science Books, Mill Valley, Calif., 1986) at pp. 532-538. U.S. Pat. No. 4,749,842 issued to Kane et al. on June 7, 1988 describes a laser diode-pumped, monolithic, unidirectional, ring laser wherein the lasant material is in thermal contact with a heating element. This patent discloses that the output wavelength of a monolithic, solid-state, unidirectional, ring laser can be temperature tuned by means of a heating element.
The elimination of spatial hole burning by generating circularly polarized light in the lasant material is described by V. Evtuhov et al., Appl. Optics, Vol. 4, No. 1, pp. 142-143 (1965). In this approach, an axially uniform energy density is created in the lasant material by forcing the laser mode to be a circularly polarized standing wave. The standing-wave electric field vector of such a mode changes in direction but not in magnitude as a function of position in the lasant material. Accordingly, there are no electric field nodes to cause spatial hole burning. A laser of this type is commonly referred to as a "twisted-mode" device.
A twisted-mode, single-frequency, neodymium-doped YAG laser has been reported by D. A. Draegert, IEEE J. Quantum Electronics, Vol. QE-8, No. 2, Feb. 1972, pp. 235-239. The laser described in this report was optically-pumped with a tungsten-iodine lamp and contained within its optical cavity a neodymium-doped YAG rod positioned between two quarter-wave plates together with a Brewster's angle plate placed immediately in front of one of the two end mirrors of the cavity. The fast axes of the two quarter-wave plates were perpendicular to each other and oriented at a 45.degree. angle to the direction of polarization determined by the Brewster's angle plate. In addition, it is stated that this twisted-mode technique should work well with small diode-pumped neodymium-doped YAG lasers. More recently, a twisted-mode, single-frequency, laser diode-pumped, neodymium-doped YAG laser has been described at pp. 38-40 of Laser Focus World (Apr. 1989).
It is known in the art that amplitude and frequency instabilities can be observed in the output of single-frequency solid-state lasers. For example, such instabilities are discussed by Danielmeyer, IEEE J. Quantum Electronics, Vol. QE-6, No. 2, Feb. 1970, pp. 101-104.